This invention deals generally with condition responsive indicating systems based on disturbances of magnetic fields and more specifically with ferromagnetic object detectors such as vehicle detectors.
Use of the earth's magnetic field as a basis for detection of ferromagnetic objects such as vehicles is not new. U.S. Pat. No. 3,237,155 to Brockett discloses such a system which is typical of many prior art devices that use an amplifier operating at the base band frequency of the signal input along with classical frequency filtering to attempt to overcome the several sources of electrical noise which can affect such a circuit.
The input signal for such devices is generated by the distortion of the earth's magnetic field which occurs when a large ferromagnetic object such as a vehicle moves within the magnetic field. The vehicle actually bends the magnetic lines of force near it as it concentrates those lines into its ferromagnetic body. This distortion moves along with the vehicle and can best be envisioned as an electromagnetic wave motion which causes a changing magnetic field in any location as the vehicle approaches and departs from the location. A coil of wire located in such a region with a changing magnetic field has a current generated in it and a corresponding voltage developed across it in accordance with the laws of electromagnetic induction.
The important characteristics of the currents generated in a passive sensing coil by the motion of a vehicle through the earth's magnetic field are that the voltages are very low frequency and very low amplitude, and these are the factors which have made prior art vehicle detector circuits unreliable. Weak signals are always subject to problems with extraneous electrical noise. Such noise may be internally generated by the circuit components themselves or from external sources such as power lines, lightning, and radio frequency generators, and even though band pass frequency filters have traditionally been used in vehicle detection circuits, such electrical noise can actually enter the circuits, or be generated within them, at locations throughout the band pass filter and amplifier stages.
The noise problem is made more difficult by the very low frequency generated by the magnetic sensing coils. The typical input frequency for vehicle detection circuits is between 0.1 Hz and 5 Hz, and the typical amplifiers used for such circuits are band pass amplifiers operated at the base band frequency of the sensing coil output and having a practical bandwidth of 0.5 Hz to 5.0 Hz. The lower end frequency of such amplifiers is limited because the overwhelming effects of low frequency noise and temperature drift generated within the circuit components have increasing effect below 1 Hz. Also internally present within the circuits are spurious noise transients, and all these noise and drift factors combine with externally generated noise. The base band approach also has problems in that all these noise components manifest themselves in oscillation or "ringing" of the band pass amplifiers at the base band frequency. Since this frequency is similar to that produced by the sensing coil, it becomes extremely difficult to distinguish such noise generated false signals from true detection.
Since such low frequency amplifiers are AC coupled, they require very large capacitors which also add to the noise problems because of internal leakage. Another problem is that the physical size of the circuits can not be reduced below the substantial size of the capacitors. A more subtle difficulty encountered due to the large capacitors is slow amplifier power up times, because of the charging times required for such large capacitors. This means that the amplifiers must be left on continuously which results in high power consumption. An unsatisfactory compromise is to reduce the size of the capacitors by discarding the lowest of the frequencies generated by the magnetic coil sensor.